Not Applicable
Not Applicable
Not Applicable
The present invention relates to fluid pressure devices, and more particularly, to such devices which are used primarily in closed loop hydraulic circuits, wherein the fluid pressure device includes a shuttle valve arrangement.
Although it should become apparent from the subsequent description that the invention may be useful with many types of fluid pressure devices, it is especially advantageous when used with a low-speed, high torque hydraulic motor, and will be described in connection therewith. Furthermore, although the invention may be used with fluid pressure devices having various types of displacement mechanisms, the invention is especially useful in a device including a gerotor displacement mechanism and will be described in connection therewith.
The use of low-speed, high-torque (LSHT) gerotor motors is becoming increasingly common in closed loop hydraulic circuits, i.e., a circuit in which the outlet port of the motor is connected directly to the inlet port of the pump, rather than to the system reservoir. This is especially true in regard to mobile applications, such as agricultural and construction equipment in which the gerotor motor is used to propel the vehicle wheels.
In closed loop propel circuits of the type described, it is frequently necessary to divert a portion of the return fluid flow (i.e., the flow from the motor outlet to the pump inlet), and pass the diverted flow through a heat exchanger to prevent overheating of the system fluid. This is normally accomplished by means of a shuttle valve assembly (xe2x80x9chot oil shuttlexe2x80x9d) installed in the motor to provide fluid communication between the low pressure (return) side of the motor and a shuttle port. The shuttle port is then connected by means of a cooler line to the inlet of the heat exchanger, and after the diverted fluid flows through the heat exchanger, it then flows to the pump inlet.
Typical, prior art hot oil shuttle designs require that the inlet pressure be approximately twice the return side pressure, in order to cause the shuttle assembly to shift from a centered (closed) position to an open position and permit flow from the return side to the shuttle port. The prior art two-to-one relationship was not a problem when the main system pump was controlled manually, and the charge pump maintained the low (return) side of the loop at about 200 psi. In that case, when the fluid entering the motor inlet port reached about 400 psi, the hot oil shuttle would shift and begin to cool the system fluid.
However, more recently, many of the pumps used in propel applications, instead of being controlled manually, have had their displacement controlled by a fluid pressure operated servo. A typical servo might require a pressure in the range of about 400 psi for proper operation, such that the charge pump in such a system must maintain the low pressure side of the loop at 400 psi. As a result, using the prior art hot oil shuttle, operating on the above-described two-to-one relationship, means that the shuttle valve will not open until the fluid on the high pressure side of the loop reaches about 800 psi.
In many applications of such a propel system as described above, the system could operate for an extended period of time without the system pressure reaching the pressure level required to open the prior art hot oil shuttle. The result would be overheating of the system fluid, and potentially, substantial damage to various parts of the closed loop hydrostatic system.
Accordingly, it is an object of the present invention to provide an improved fluid pressure device of the type including a shuttle valve assembly which overcomes the above-described problems of the prior art.
It is a more specific object of the present invention to provide a shuttle valve assembly which is able to open at substantially less than a two-to-one relationship of inlet pressure (high pressure) to return pressure (low pressure).
It is an even more specific object of the present invention to provide such an improved shuttle valve assembly, which accomplishes the above-stated objects, and in which the relatively elevated return pressure is used to assist in the movement of the shuttle valve poppet (i.e., the one on the low pressure side) from its closed position to its open position.
The above and other objects of the present invention are accomplished by the provision of an improved fluid pressure device including a housing defining a high pressure fluid port and a low pressure fluid port. The housing further defines a shuttle bore including a high pressure chamber in fluid communication with the high pressure port and a low pressure chamber in fluid communication with the low pressure port and a shuttle outlet port in fluid communication with the shuttle bore at a location axially intermediate the high and low pressure chambers. The shuttle bore defines a first valve seat disposed adjacent the high pressure chamber and a second valve seat disposed adjacent the low pressure chamber. A shuttle assembly is disposed in the shuttle bore including a shuttle spool reciprocably disposed in the shuttle bore, a first poppet and means biasing the first poppet into engagement with the first valve seat, and a second poppet and means biasing the second poppet into engagement with the second valve seat when the shuttle spool is in a centered range of positions. The shuttle spool extends axially through, and is surrounded by, each of the poppets and includes engagement means operable to engage and move the second poppet out of engagement with the second valve seat as the shuttle spool moves from the centered range of positions to a first activated position in response to high pressure fluid in the high pressure chamber.
The improved fluid pressure device is characterized by the shuttle spool defining a fluid passage disposed to provide fluid communication from the low pressure chamber to the shuttle outlet port, as the shuttle spool approaches the end of the centered range of positions. As a result, fluid pressure in the low pressure chamber is communicated to the shuttle bore surrounding the shuttle spool and acts on the second poppet, in opposition to the means biasing the second poppet, as the engagement means engages and moves the second poppet out of engagement with the second valve seat.